The Sun

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Our Sun is a 4.5 billion-year-old star – a hot glowing ball of hydrogen and helium at the center of our solar system. The Sun is about 93 million miles (150 million kilometers) from Earth, and without its energy, life as we know it could not exist here on our home planet.

The Sun is the largest object in our solar system. The Sun’s volume would need 1.3 million Earths to fill it. Its gravity holds the solar system together, keeping everything from the biggest planets to the smallest bits of debris in orbit around it. The hottest part of the Sun is its core, where temperatures top 27 million degrees Fahrenheit (15 million degrees Celsius). The Sun’s activity, from its powerful eruptions to the steady stream of charged particles it sends out, influences the nature of space throughout the solar system.

1. Orbit and Rotation

The Sun orbits the center of the Milky Way, bringing with it the planets, asteroids, comets, and other objects in our solar system. Our solar system is moving with an average velocity of 450,000 miles per hour (720,000 kilometers per hour). But even at this speed, it takes about 230 million years for the Sun to make one complete trip around the Milky Way.

The Sun rotates on its axis as it revolves around the galaxy. Its spin has a tilt of 7.25 degrees with respect to the plane of the planets’ orbits. Since the Sun is not solid, different parts rotate at different rates. At the equator, the Sun spins around once about every 25 Earth days, but at its poles, the Sun rotates once on its axis every 36 Earth days.

2. Formation

The Sun formed about 4.6 billion years ago in a giant, spinning cloud of gas and dust called the solar nebula. As the nebula collapsed under its own gravity, it spun faster and flattened into a disk. Most of the nebula\’s material was pulled toward the center to form our Sun, which accounts for 99.8% of our solar system’s mass. Much of the remaining material formed the planets and other objects that now orbit the Sun. (The rest of the leftover gas and dust was blown away by the young Sun\’s early solar wind.)

Like all stars, our Sun will eventually run out of energy. When it starts to die, the Sun will expand into a red giant star, becoming so large that it will engulf Mercury and Venus, and possibly Earth as well. Scientists predict the Sun is a little less than halfway through its lifetime and will last another 5 billion years or so before it becomes a white dwarf.

3. Structure

The Sun is a huge ball of hydrogen and helium held together by its own gravity.

The Sun has several regions. The interior regions include the core, the radiative zone, and the convection zone. Moving outward – the visible surface or photosphere is next, then the chromosphere, followed by the transition zone, and then the corona – the Sun’s expansive outer atmosphere.

Once material leaves the corona at supersonic speeds, it becomes the solar wind, which forms a huge magnetic \”bubble\” around the Sun, called the heliosphere. The heliosphere extends beyond the orbit of the planets in our solar system. Thus, Earth exists inside the Sun’s atmosphere. Outside the heliosphere is interstellar space.

The core is the hottest part of the Sun. Nuclear reactions here – where hydrogen is fused to form helium – power the Sun’s heat and light. Temperatures top 27 million °F (15 million °C) and it’s about 86,000 miles (138,000 kilometers) thick. The density of the Sun’s core is about 150 grams per cubic centimeter (g/cm³). That is approximately 8 times the density of gold (19.3 g/cm³) or 13 times the density of lead (11.3 g/cm³).

Energy from the core is carried outward by radiation. This radiation bounces around the radiative zone, taking about 170,000 years to get from the core to the top of the convection zone. Moving outward, in the convection zone, the temperature drops below 3.5 million °F (2 million °C). Here, large bubbles of hot plasma (a soup of ionized atoms) move upward toward the photosphere, which is the layer we think of as the Sun\’s surface.

4. Surface

The Sun doesn’t have a solid surface like Earth and the other rocky planets and moons. The part of the Sun commonly called its surface is the photosphere. The word photosphere means \”light sphere\” – which is apt because this is the layer that emits the most visible light. It’s what we see from Earth with our eyes. (Hopefully, it goes without saying – but never look directly at the Sun without protecting your eyes.)

Although we call it the surface, the photosphere is actually the first layer of the solar atmosphere. It\’s about 250 miles thick, with temperatures reaching about 10,000 degrees Fahrenheit (5,500 degrees Celsius). That\’s much cooler than the blazing core, but it\’s still hot enough to make carbon – like diamonds and graphite – not just melt, but boil. Most of the Sun\’s radiation escapes outward from the photosphere into space.

5. Atmosphere

Above the photosphere is the chromosphere, the transition zone, and the corona. Not all scientists refer to the transition zone as its own region – it is simply the thin layer where the chromosphere rapidly heats and becomes the corona. The photosphere, chromosphere, and corona are all part of the Sun’s atmosphere. (The corona is sometimes casually referred to as “the Sun’s atmosphere,” but it is actually the Sun’s upper atmosphere.)

The Sun’s atmosphere is where we see features such as sunspots, coronal holes, and solar flares.

6. Key Sun Features

6.1. Sunspots

They look like dark holes in the Sun, but they are actually areas that are slightly cooler than the surrounding photosphere. Sunspots are created where bits of the Sun\’s magnetic field poke out from the interior into the Sun\’s atmosphere. Lasting from days to months, sunspots range in size from 1,000 to 100,000 miles (1,600 to 160,900 kilometers).

6.2. Coronal Holes

A coronal hole is a patch of the Sun’s atmosphere with much lower density than the surrounding areas. In ultraviolet views of the Sun, coronal holes appear as dark splotches. These regions are where the Sun’s magnetic field lines are connected directly to interplanetary space, allowing solar material to escape out in a high-speed stream of solar wind, leaving a dark “hole” near the surface of the Sun.

6.3. Solar Flares

Solar flares are tremendously energetic bursts of light and particles triggered by the release of magnetic energy on the Sun. Flares are by far the most powerful explosions in the solar system, with energy releases comparable to billions of hydrogen bombs

6.4. Coronal Mass Ejection (CME)

Coronal mass ejections, or CMEs, are immense clouds of magnetized particles blasted into space by the Sun at over a million miles per hour, often following after a solar flare. CMEs expand as they sweep through space, often measuring millions of miles across. When directed at Earth, a CME can produce geomagnetic disturbances that ignite bright auroras, short-circuit satellites, and power grids on Earth, or at their worst, even endanger astronauts in orbit.

6.5. Solar Prominence

A prominence is a snakelike structure made of cooler, denser solar material that is suspended above the Sun’s surface by a strong local magnetic field. (When they are viewed against the solar disk, head-on rather than off the visible edge, they are called filaments.) Prominences can erupt when the magnetic structure becomes unstable, flinging their plasma outward in a blast called a coronal mass ejection.

6.6. Spicules

At any given moment, as many as 10 million wild jets of solar material burst up from the Sun’s surface. Known as spicules, these grass-like tendrils of plasma erupt as fast as 60 miles per second (100 kilometers per second) and can reach lengths of 6,000 miles (9,700 kilometers) before collapsing.

7. What are Lagrange Points?

Lagrange Points are positions in space where the gravitational forces of a two-body system like the Sun and Earth produce enhanced regions of attraction and repulsion. These can be used by spacecraft as \”parking spots\” in space to remain in a fixed position with minimal fuel consumption.

There are five special points where a small mass can orbit in a constant pattern with two larger masses. The Lagrange Points are positions where the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them. This mathematical problem, known as the \”General Three-Body Problem\” was considered by Italian-French mathematician Joseph-Louis Lagrange in his prize-winning paper (Essai sur le Problème des Trois Corps, 1772).

Of the five Lagrange points, three are unstable and two are stable. The unstable Lagrange points – labeled L1, L2, and L3 – lie along the line connecting the two large masses. The stable Lagrange points – labeled L4 and L5 – form the apex of two equilateral triangles that have the large masses at their vertices. L4 leads the orbit of earth and L5 follows.

The L1 point of the Earth-Sun system affords an uninterrupted view of the sun and is currently home to the Solar and Heliospheric Observatory Satellite SOHO. The L2 point of the Earth-Sun system was the home to the WMAP spacecraft, current home of Planck, and future home of the James Webb Space Telescope. L2 is ideal for astronomy because a spacecraft is close enough to readily communicate with Earth, can keep Sun, Earth and Moon behind the spacecraft for solar power and (with appropriate shielding) provides a clear view of deep space for our telescopes. The L1 and L2 points are unstable on a time scale of approximately 23 days, which requires satellites orbiting these positions to undergo regular course and attitude corrections.

NASA is unlikely to find any use for the L3 point since it remains hidden behind the Sun at all times. The idea of a hidden \”Planet-X\” at the L3 point has been a popular topic in science fiction writing. The instability of Planet X\’s orbit (on a time scale of 150 years) didn\’t stop Hollywood from turning out classics like The Man from Planet X.

The L4 and L5 points are home to stable orbits so long as the mass ratio between the two large masses exceeds 24.96. This condition is satisfied for both the Earth-Sun and Earth-Moon systems, and for many other pairs of bodies in the solar system. Objects found orbiting at the L4 and L5 points are often called Trojans after the three large asteroids Agamemnon, Achilles and Hector that orbit in the L4 and L5 points of the Jupiter-Sun system. (According to Homer, Hector was the Trojan champion slain by Achilles during King Agamemnon\’s siege of Troy).

There are hundreds of Trojan Asteroids in the solar system. Most orbit with Jupiter, but others orbit with Mars. In addition, several of Saturn\’s moons have Trojan companions. In 1956 the Polish astronomer Kordylewski discovered large concentrations of dust at the Trojan points of the Earth-Moon system. The DIRBE instrument on the COBE satellite confirmed earlier IRAS observations of a dust ring following the Earth\’s orbit around the Sun. The existence of this ring is closely related to the Trojan points, but the story is complicated by the effects of radiation pressure on the dust grains. In 2010 NASA\’s WISE telescope finally confirmed the first Trojan asteroid (2010 TK7) around Earth\’s leading Lagrange point.

7.1. Finding the Lagrange Points

The easiest way to understand Lagrange points is to adopt a frame of reference that rotates with the system. The forces exerted on a body at rest in this frame can be derived from an effective potential in much the same way that wind speeds can be inferred from a weather map. The forces are strongest when the contours of the effective potential are closest together and weakest when the contours are far apart.

In the above contour plot we see that L4 and L5 correspond to hilltops and L1, L2 and L3 correspond to saddles (i.e. points where the potential is curving up in one direction and down in the other). This suggests that satellites placed at the Lagrange points will have a tendency to wander off (try sitting a marble on top of a watermelon or on top of a real saddle and you get the idea). But when a satellite parked at L4 or L5 starts to roll off the hill it picks up speed. At this point the Coriolis force comes into play – the same force that causes hurricanes to spin up on the earth – and sends the satellite into a stable orbit around the Lagrange point.

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